1. Field of the Invention
The invention relates to an epitaxially coated silicon wafer and to a method for producing epitaxially coated silicon wafers.
2. Background Art
Epitaxially coated silicon wafers are suitable for use in the semiconductor industry, in particular for the fabrication of large scale integrated electronic components such as microprocessors or memory chips. In this case, stringent requirements are made of the flatness of the front sides of the silicon wafers on which the electronic components are produced. This is necessary in order to avoid problems during exposure of the silicon wafers (lithography) and in intermediate polishing processes (“Chemical Mechanical Polishing”, CMP) during fabrication of the components.
A critical property in this case is the local flatness or local geometry of the silicon wafer on its front side. Modern stepper technology requires optimum local planarities in partial regions of the front side of the silicon wafer, expressed for example as SFQR “site front-surface referenced least squares/range” which is magnitude of the positive and negative deviation from a front side defined by minimizing the square error for a component area (measurement window, “site”) of defined size. The maximum local flatness value SFQRmax specifies the maximum SFQR value for the component areas taken into account on a silicon wafer.
The maximum local flatness value is usually determined taking account of an edge exclusion of 3 mm, by way of example. An area on a silicon wafer within a nominal edge exclusion is usually referred to as “Fixed Quality Area”, or to FQA. Those sites which have part of their area lying outside the FQA, but the center of which lies within the FQA, are called “partial sites”. The determination of the maximum local flatness often does not involve using the “partial sites”, but rather only the so-called “full sites”, that is to say the component areas lying completely within the FQA. In order to be able to compare maximum local flatness values, it is essential to specify the edge exclusion and thus the size of the FQA and furthermore to specify whether or not the “partial sites” have been taken into account.
A generally recognized rule of thumb states that the SFQRmax value of a silicon wafer must be less than or equal to the possible line width of semiconductor components that are to be produced on the silicon wafer. If this value is exceeded, the stepper experiences focusing problems and the component in question is thus lost. With regard to optimizing costs, however, it is customary nowadays not to reject a silicon wafer owing, for example, only to a component area that exceeds the SFQRmax value specified by the component manufacturer, but rather to permit a defined percentage, usually 1%, of component areas with higher values. The percentage of the sites which are permitted to lie below a specific limit value of a geometry parameter is usually specified by a PUA (“Percent Useable Area”) value, which, e.g. in the case of an SFQRmax of less than or equal to 0.1 μm and a PUA value of 99%, means that 99% of the sites have an SFQRmax of less than or equal to 0.1 μm, while higher SFQR values are permitted for 1% of the sites (“chip yield”).
According to the prior art, a silicon wafer can be produced by a process sequence of separating a single crystal of silicon into wafers, rounding the mechanically sensitive edges, carrying out an abrasive step such as grinding or lapping followed by a polishing. EP 547894 A1 describes a lapping method; grinding methods are disclosed in published applications EP 272531 A1 and EP 580162 A1.
The final flatness is generally produced by the polishing process, which may be preceded, if appropriate, by an etching step for removing disturbed crystal layers and for removing impurities. A suitable etching method is known from DE 19833257 C1, by way of example. Because traditional single-side polishing methods generally lead to poorer plane-parallelism, polishing methods acting on both sides (“double-side polishing”) have been developed which make it possible to produce silicon wafers with improved flatness. In the case of polished silicon wafers, therefore, the required flatness is achieved by mechanical and chemo-mechanical processing steps such as grinding, lapping and polishing.
DE 19938340 C1 describes providing monocrystalline silicon wafers with a layer grown in monocrystalline fashion and made of silicon with the same crystal orientation, a so-called epitaxial coating, on which semiconductor components are later fabricated. Epitaxially coated silicon wafers of this type have certain advantages over silicon wafers made of homogeneous material, for example preventing charge reversal in bipolar CMOS circuits followed by short circuiting of the component (“latch-up”), lower defect densities (for example a reduced number of COPs (“crystal-originated particles”), and also the absence of an appreciable oxygen content, whereby it is possible to preclude a short-circuit risk due to oxygen precipitates in component-relevant regions.
According to the prior art, epitaxially coated silicon wafers are produced from suitable intermediates usually by means of a process sequence of removal polishing—final polishing—cleaning—epitaxy.
DE 10025871 A1 discloses a method for producing a silicon wafer with an epitaxial layer deposited on the front side, this method comprising the following process steps:
(a) a removal polishing as sole polishing step;
(b) hydrophilic cleaning and drying of the silicon wafer;
(c) pretreatment of the front side of the silicon wafer at a temperature of 950 to 1250 degrees Celsius in an epitaxy reactor; and
(d) deposition of an epitaxial layer on the front side of the pretreated silicon wafer.
It is customary, in order to protect silicon wafers from particle loading, to subject the silicon wafers to a hydrophilic cleaning after polishing. The hydrophilic cleaning produces a native oxide on the silicon wafers which is very thin, for example approximately 0.5-2 nm in thickness, depending on the type of cleaning and measurement. The native oxide is later removed later in the course of a pretreatment of the silicon wafer in an epitaxy reactor, usually under a hydrogen atmosphere (also called “H2-Bake”).
In a second step, the surface roughness of the front side of the silicon wafer is reduced and polishing defects are removed from the surface by the silicon wafer being pretreated with an etching medium. Gaseous hydrogen chloride (HCl) is usually used as the etching medium and added to the hydrogen atmosphere (“HCl etchant”).
The silicon wafer that has been pretreated in this way subsequently acquires an epitaxial layer. Epitaxy reactors, which are used in particular in the semiconductor industry for the deposition of an epitaxial layer on a silicon wafer, are described in the prior art. For this purpose, in the epitaxy reactor, one or more silicon wafers are heated by means of heating sources, preferably by means of upper and lower heating sources, for example lamps or lamp banks, and subsequently exposed to a gas mixture, comprising a source gas comprising a silicon compound (silanes), a carrier gas for example hydrogen and, if appropriate, a doping gas, for example diborane.
The epitaxial layer is usually deposited according to the CVD method (“chemical vapor deposition”) by a procedure in which silanes, for example trichlorosilane (SiHCl3, TCS), are passed as the source gas to the surface of the silicon wafer, decompose there at temperatures of 600 to 1250° C. to form elemental silicon and volatile byproducts, and thus form an epitaxially grown silicon layer on the silicon wafer. The epitaxial layer may be undoped, or may be doped in a targeted manner with boron, phosphorus, arsenic or antimony by means of suitable doping gases, in order to set the conduction type and conductivity.
A susceptor, which comprises graphite, silicon carbide (SiC) or quartz, for example, and which is situated in the deposition chamber of the epitaxy reactor, serves as a support for the silicon wafer during the pretreatment steps and during the epitaxial coating. In this case, the silicon wafer usually rests in milled-out portions of the susceptor in order to ensure a uniform heating and to isolate the rear side of the silicon wafer, on which generally no layer is deposited, from the source gas.
In accordance with the prior art, the process chambers of the epitaxy reactors are designed for one or more silicon wafers. In the case of silicon wafers having relatively large diameters (greater than or equal to 150 mm), single wafer reactors are usually used since the latter are known for their good epitaxial layer thickness regularity. The uniformity of the layer thickness can be established by various measures, for example by altering the gas flows (hydrogen, TCS), by incorporating and adjusting gas inlet devices (injectors), by changing the deposition temperature, or by alterations to the susceptor.
In epitaxy, it is customary, following a number of epitaxial depositions on silicon wafers, to carry out an etching treatment of the susceptor without a substrate, in the course of which the susceptor and also other parts of the process chamber are freed of silicon deposits. This etching treatment, which may be effected using hydrogen chloride (HCl), for example, is often performed after the processing of only a small number of silicon wafers (e.g. after 3-5 epitaxial coatings) in the case of single wafer reactors, and is not carried out until after the processing of a higher number of silicon wafers (e.g. after 10-20 epitaxial coatings) in the case of depositing thin epitaxial layers.
Usually, only an etching treatment using HCl or else an etching treatment using HCl followed by brief coating of the susceptor is carried out. The coating of the susceptor is effected in order that the silicon wafer does not bear directly on the susceptor.
It has been shown that the methods known in the prior art lead to a poor yield, since portions of the epitaxially coated silicon wafers have poor local flatness values, particularly in the edge region. By way of example, if an etching treatment of the susceptor is carried out after four epitaxial depositions in each case, at least one of said epitaxially coated silicon wafers will have significantly poorer local flatness values in the edge region. The maximum local flatness parameter SFQRmax is usually in the region of 0.05 μm or higher for these epitaxially coated silicon wafers, for which reason they are unsuitable for the imaging of structures (stepper technology) for the future generation of electronic components having line widths of less than 45 nm.